1. Field of the Invention
This invention relates to the use of hydroxyalkyl polyethylene glycol ethers in rinse aids for dishwashing machines.
In machine dishwashing, there are generally two rinse cycles in which different detergents are used and which are mostly separated by intermediate rinses with clean water. Alkaline detergents are used in the actual wash cycle to detach and emulsify food remains left on the dishes. By contrast, special rinse aids are used in the so-called final-rinse cycle. Rinse aids are not detergents, but are intended to have good wetting power and to reduce the surface tension of the final-rinse water to such an extent that it drains from the dishes like a film and leaves behind no visible residues which could lead to lime stains or other marks.
On account of the vigorous liquor movement both in domestic and in institutional dishwashing machines, the rinse aids have to be low-foam formulations. Conventional anionic wetting agents, such as relatively high molecular weight alkyl sulfates or alkyl or alkylaryl sulfonates, show a pronounced tendency towards foaming so that they are not suitable for use as rinse aids. Rinse aids based on nonionic surfactants, for example ethylene oxide adducts with fatty alcohols, alkylphenols or polypropylene glycols of relatively high molecular weight, are now widely used in practice.
However, it has been found that, in general, rinse aids containing nonionic surfactants such as these also produce too much foam in the concentration ranges required for an adequate wetting effect. They easily lead to problems in the machines through excessive and stable foaming. This is due to the fact, above all in institutional dishwashing machines with very intensive water circulation, that the final-rinse liquor which has a temperature of about 80.degree. C. is returned to the main wash cycle where it comes into contact with the alkaline detergents at 50.degree. to 70.degree. C. In these zones of the dishwashing machines, foaming is further promoted by food remains present in the wash liquor. Although difficulties such as these arise on only a reduced scale in domestic dishwashing machines, they occur in principle to the same extent.
Accordingly, it is necessary to add foam inhibitors to the rinse aids, even where relatively low-foam ethylene oxide adducts are used. Suitable foam initiators include nonionic alkoxylation products which are poorly soluble in water at final-rinse temperatures. Products such as these include ethylene oxide adducts with higher alcohols, alkylphenols or amines having a low degree of ethoxylation or corresponding adducts of ethylene oxide and propylene oxide or propylene oxide and ethylene oxide in any order and in any ratio. However, compounds such as these do not have any wetting effect at in-use temperatures and, accordingly, are a burden on the rinse aid.
2. Discussion of Related Art
However, extremely low-foam, biodegradable rinse aids which exclusively contain constituents acting as wetting agents and which make it unnecessary to add foam-inhibiting ballast surfactants or other foam inhibitors are also known from German Patent 21 06 819. The rinse aids therein are for dishwashing machines based on nonionic, low-foam surfactants containing adducts of 5 to 20 mol ethylene oxide and 1 to 10 mol propylene oxide with secondary aliphatic alcohols containing a linear C.sub.10 -C.sub.20 alkyl chain. The adducts mentioned exhibit an excellent draining and clear drying effect both on standard items such as china plates, cutlery and on glasses which are particularly difficult to rinse in the final rinse cycle. The biodegradability requirements prevailing at that time were satisfied. However, the compounds mentioned do not satisfy current legal requirements on the biodegradability of compounds entering the wastewater.
Hydroxyalkyl polyethylene glycol ethers corresponding to the following general formula ##STR2## in which R.sup.1 is a linear C.sub.6 -C.sub.16 alkyl radical,
R.sup.2 is a linear or branched C.sub.4 -C.sub.8 alkyl radical, PA0 R.sup.3 is hydrogen or a C.sub.1 -C.sub.8 alkyl radical and PA0 n is a number of 7 to 12, PA0 R.sup.2 is a linear or branched C.sub.4 -C.sub.8 alkyl radical, PA0 R.sup.3 is hydrogen or a C.sub.1 -C.sub.16 alkyl radical and PA0 n is a number of 7 to 30,
are known from German patent application 33 45 349. The use of these compounds as foam-inhibiting additives in low-foam cleaning preparations is also described in this publication. The principal advantage of the described compounds (I) is not only their pronounced low-foam character, but also their high stability to acids and, in particular, alkalis which is absolutely essential for cleaning preparations on account of the high alkali concentrations generally present in cleaning preparations. In addition, the described compounds (I) possess extremely good biodegradability which even satisfies current legal requirements.